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Britannica Illustrated Science Library
Britannica Illustrated Science Library
UNIVERSE
UNIVERSE
© 2
008 Editorial Sol 90
All rights reserved.

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Universe

Contents
PICTURE ON PAGE 1
Image of a planetary nebula.
Planetary nebulae are among
the most photogenic objects
in astronomy.
What Is the
Universe?
Page 6
What Is in the
Universe?
Page 18
The Solar
System
Page 38
The Earth
and the Moon
Page 66
Observing
the Universe
Page 80
T
here was a time when people believed
that the stars were bonfires lit by
other tribes in the sky, that the
universe was a flat plate resting on the shell
of a giant turtle, and that the Earth,
according to the Greek astronomer Ptolemy,
was at the center of the universe. From the
most remote of times, people have been

curious about what lies hidden beyond the
celestial sphere. This curiosity has led them
to build telescopes that show with clarity
otherwise blurry and distant objects. In this
book you will find the history of the cosmos
illustrated with spectacular images that
show in detail how the cosmos was formed,
the nature of the many points of light that
adorn the night sky, and what lies ahead.
You will also discover how the suns that
inhabit space live and die, what dark matter
and black holes are, and what our place is in
this vastness. Certainly, the opportunity to
compare the destiny of other worlds similar
to ours will help us understand that for the
time being there is no better place than the
Earth to live. At least for now.
I
n the Milky Way—according to
mathematical and physical
calculations—there are more than 100
billion stars, and such a multitude leads to
the question: Is it possible that our Sun is
the only star that possesses an inhabited
planet? Astronomers are more convinced
than ever of the possibility of life in other
worlds. We just need to find them. Reading
this book will let you become better
acquainted with our neighbors in the solar
system—the other planets—and the most

important characteristics that distinguish
them. All this information that explores the
mysteries of space is accompanied by
recent images captured by the newest
telescopes. They reveal many details about
the planets and their satellites, such as the
volcanoes and craters found on the surface
of some of them. You will also learn more
about the asteroids and comets that orbit
the Sun and about Pluto, a dwarf planet,
which is to be visited by a space probe for
the first time. Less than a decade ago,
astronomers began observing frozen worlds,
much smaller than a planet, in a region of
the solar system called the Kuiper belt. We
invite you to explore all of this. The images
and illustrations that accompany the text
will prove very helpful in studying and
understanding the structure of all the visible
and invisible objects (such as dark matter)
that form part of the universe. There are
stellar maps showing the constellations, the
groups of stars that since ancient times have
served as a guide for navigation and for the
development of calendars. There is also a
review through history: from Ptolemy, who
thought the planets orbited around the
Earth, and Copernicus, who put the Sun in
the center, and Galileo, the first to aim a
telescope skyward, up to the most recent

astronomical theories, such as those of
Stephen Hawking, the genius of space and
time who continues to amaze with his
discoveries about the greatest mysteries of
the cosmos. You will find these and many
more topics no matter where you look in this
fantastic book that puts the universe and its
secrets in your hands.
The Secrets of
the Universe
CONE NEBULA
This nebula got its name
from its cone shape, as
shown in the image.
What Is the Universe?
T
he universe is everything that
exists, from the smallest
particles to the largest ones,
together with all matter and
energy. The universe includes
visible and invisible things, such as dark
matter, the great, secret component of
the cosmos. The search for dark matter
is currently one of the most important
tasks of cosmology. Dark matter may
literally determine the density of all of
space, as well as decide the destiny of
the universe. Did you know that, second
by second, the universe grows and

grows? The question that astronomers
are asking—the question that concerns
them the most—is how much longer the
universe can continue to expand like a
balloon before turning into something
cold and dark.
X-RAY OF THE COSMOS 8-9
THE INSTANT OF CREATION 10-13
EVERYTHING COMES TO AN END 14-15
THE FORCES OF THE UNIVERSE 16-17
DARK MATTER
Evidence exists that dark matter, though invisible
to telescopes, betrays itself by the gravitational
pull it exerts over other heavenly bodies.
UNIVERSE
98
WHAT IS THE UNIVERSE?
T
he universe, marvelous in its majesty, is an ensemble of a hundred
billion galaxies. Each of these galaxies (which tend to be found in
large groups) has billions of stars. These galactic concentrations
surround empty spaces, called cosmic voids. The immensity of the
cosmos can be better grasped by realizing that the size of our fragile
planet Earth, or even that of the Milky Way, is insignificant
compared to the size of the remainder of the cosmos.
Originating nearly 14 billion years ago
in an immense explosion, the universe
today is too large to be able to conceive. The
innumerable stars and galaxies that populate it
promise to continue expanding for a long time.

Though it might sound strange today, for many
years, astronomers thought that the Milky Way,
where the Earth is located, constituted the entire
universe. Only recently—in the 20th century—was outer
space recognized as not only much vaster than previously
thought but also as being in a state of ongoing expansion.
The Universe
NEAR STARS Found closer
than 20 light-years from the
Sun, they make up our solar
neighborhood.
2.
NEIGHBORS Within a space
of one million light-years,
we find the Milky Way and
its closest galaxies.
3.
NEAREST GALAXIES. At a scale
of one hundred million light-years,
the galactic clusters nearest to
the Milky Way can be seen.
5.
FILAMENTS. From five billion
light-years away, the immensity of
the cosmos is evident in its
galactic filaments, each one home
to millions and millions of galaxies.
7.
SUPERCLUSTERS. Within a
distance of a billion light-years,

groups of millions of galaxies,
called superclusters, can be seen.
6.
LOCAL GROUP. Ten
million light-years away
is Andromeda, the
closest to the Earth.
4.
EARTH Originated, together
with the solar system, when
the universe was already 9.1
billion years old. It is the only
known planet that is home to life.
1.
EARTH
Neptune
G51-15
Ross
128
Lalande
21185
Wolf
359
Luyten’s
Star
Procyon
Uranus
Saturn
Jupiter
Pluto

SUN
Alpha
Centauri
Sirius
270°
90°
180°
0.5
0°
180°
0°
12.5
Epsilon
Eridani
L372-58
L726-8
L725-32
Epsilon
Indi
Lacaille
9352
Ceti
7.5
2.5
Struve
2398
Ross
248
Ross
154

Groombridge
34
61 Cygni
Bernard’s
Star
L789-6
L789-6
0°
Sextans
Dwarf
Ursa
Minor Dwarf
Leo A
Leo I
Leo II
Andromeda I
Sextans B
Sextans A
Antila
Dwarf
NGC
3109
Draco
Dwarf
Sagittarius
Dwarf
Tucana
Dwarf
Phoenix
Dwarf

Cetus
Dwarf
Sagittarius
Irregular
Dwarf
Aquarius
Dwarf
LGS 3
Pegasus
Dwarf
IC
1613
WLM
Canis
Major
Small
Magellanic
Cloud
Large
Magellanic
Cloud
Carina
Dwarf
MILKY WAY
MILKY WAY
NGC
6822
Triangle
Andromeda
M32

M110
NGC
185
NGC
147
IC 10
0.12
0.25
0.37
1.2
2.5
3.7
0°
180°
0°
180°
NGC
7582
NGC
6744
Capricornus
Supercluster
Pavo-Indus
Supercluster
Sculptor
Supercluster
Sculptor
Void
Pisces-Cetus
Superclusters

Pisces-Perseus
Supercluster
Coma
Supercluster
Centaurus
Supercluster
Hercules
Supercluster
Shapley
Supercluster
Boötes
Void
Leo
Supercluster
Ursa Major
Supercluster
Boötes
Supercluster
Corona Borealis
Supercluster
Hydra
Horologium
Superclusters
Columba
Supercluster
NGC
1023
NGC
2997
NGC

5128
NGC
5033
NGC
4697
12.5
25
37.5
50
1,000
750
250
Dorado
Sculptor
Maffei
M81
M101
Leo I
Canis
Ursa Major
Group
Virgo
Group
Leo III
Group
Virgo III
Group
Fornax
Cluster
Eridanus

Cluster
LOCAL
GROUP
VIRGO
Sextans
Supercluster
X-Ray of the Cosmos
100 billion
The total number of galaxies that exist,
indicating that the universe is both larger
and older than was previously thought
UNIVERSE
11
10
WHAT IS THE UNIVERSE?
Region 1 Region 3
Region 2
Region 4
Region 5
Galaxy 1 Galaxy 2
Galaxy 5
Galaxy 4
Galaxy 3
The Instant of Creation
I
t is impossible to know precisely how, out of nothing, the universe began to exist. According to the big
bang theory—the theory most widely accepted in the scientific community—in the beginning, there
appeared an infinitely small and dense burning ball that gave rise to space, matter, and energy. This
happened 13.7 billion years ago. The great, unanswered question is what caused a small dot of light—filled
with concentrated energy from which matter and antimatter were created—to arise from nothingness. In

very little time, the young universe began to expand and cool. Several billion years later, it acquired the
form we know today.
10
-43
sec 10
-12
sec 3 min
Scientists theorize that, from
nothing, something infinitely
small, dense, and hot appeared.
All that exists today was
compressed into a ball smaller than
the nucleus of an atom.
TIME
TEMPERATURE
Cosmic Inflation Theory
Although big bang theorists understood the universe as originating
in an extremely small, hot, and condensed ball, they could not
understand the reason for its staggering growth. In 1981, physicist Alan
Guth proposed a solution to the problem with his inflationary theory. In an
extremely short period of time (less than a thousandth of a second), the
universe grew more than a trillion trillion trillion times. Near the end of this
period of expansion, the temperature approached absolute zero.
HOW IT DID
NOT GROW
Had the universe not
undergone inflation,
it would be a
collection of different
regions, each with its

own particular types
of galaxies and each
clearly
distinguishable from
the others.
HOW IT GREW
Cosmic inflation was
an expansion of the
entire universe. The
Earth's galactic
neighborhood appears
fairly uniform.
Everywhere you look,
the types of galaxies
and the background
temperature are
essentially the same.
FROM PARTICLES TO MATTER
The quarks, among the oldest particles,
interact with each other by forces
transmitted through gluons. Later protons
and neutrons will join to form nuclei.
Photon
Massless elemental
luminous particle
Gluon
Responsible for
the interactions
between quarks
Quark

Light, elemental
particle
Graviton
It is believed to
transmit gravitation.
0
Gravity
SUPERFORCE
Strong nuclear
Weak nuclear
Electromagnetism
EXPANSION
10
-38
sec
10
32
° F (and C) 10
29
° F (and C)
10
15
° F (and C) 2x10
9
° F (1x10
9
° C)-
1
At the closest moment to
zero time, which physics has

been able to reach, the
temperature is extremely
high. Before the universe's inflation,
a superforce governed everything.
2
The universe is unstable. Only
10
-38
seconds after the big
bang, the universe increases in
size more than a trillion trillion
trillion times. The expansion of the
universe and the division of its forces begin.
3
The universe experiences a
gigantic cooldown. Gravity
has already become
distinguishable, and the
electromagnetic force and the strong
and weak nuclear interactions appear.
4
5 sec
9x10
9
° F (5x10
9
° C)
The electrons and their
antiparticles,
positrons, annihilate

each other until the
positrons disappear. The
remaining electrons form atoms.
6
10
-4
sec
10
12
° F (and C)
Protons and neutrons
appear, formed by three
quarks apiece. Because
all light is trapped within
the web of particles, the universe
is still dark.
5
The nuclei of the
lightest elements,
hydrogen and
helium, form.
Protons and neutrons unite to
form the nuclei of atoms.
7
WMAP (WILKINSON MICROWAVE ANISOTROPY PROBE)
NASA's WMAP project maps the background radiation of the universe. In the
image, hotter (red-yellow) regions and colder (blue-green) regions can be
observed. WMAP makes it possible to determine the amount of dark matter.
THE SEPARATION OF FORCES
Before the universe expanded, during a period of

radiation, only one unified force governed all
physical interactions. The first distinguishable
force was gravity, followed by electromagnetism
and nuclear interactions. Upon the division of the
universe's forces, matter was created.
Energetic Radiation
The burning ball that gave rise to the universe remained a
source of permanent radiation. Subatomic particles and
antiparticles annihilated each other. The ball's high density
spontaneously produced matter and destroyed it. Had this state
of affairs continued, the universe would never have undergone the
growth that scientists believe followed cosmic inflation.
1 sec
1
A gluon interacts
with a quark.
2
Quarks join by means
of gluons to form
protons and neutrons.
3
Protons and
neutrons unite to
create nuclei.
Electron
Negatively charged
elemental particle
ELEMENTARY PARTICLES
In its beginnings, the universe was a soup of particles that interacted with each other
because of high levels of radiation. Later, as the universe expanded, quarks formed the

nuclei of the elements and then joined with electrons to form atoms.
The neutrinos separate from the initial particle soup through the disintegration
of neutrons. Though having extremely little mass, the neutrinos might
nevertheless form the greatest part of the universe's dark matter.
Proton
Neutron
Quark
Gluon
UNIVERSE 13
12
WHAT IS THE UNIVERSE?
380,000 500 million
TIME
(in years)
TEMPERATURE
4,900° F (2,700° C)
-405° F (-243° C)
380,000 years after the big
bang, atoms form. Electrons
orbit the nuclei, attracted by
the protons. The universe
becomes transparent. Photons travel
through space.
8
Galaxies acquire their definitive
shape: islands of millions and
millions of stars and masses of
gases and dust. The stars explode
as supernovas and disperse heavier
elements, such as carbon.

9
FIRST ATOMS
Hydrogen and helium were the first elements to
be formed at the atomic level. They are the main
components of stars and planets. They are by far
the most abundant elements in the universe.
The vast span of time related to the history of
the universe can be readily understood if it is
scaled to correspond to a single year—a year
that spans the beginning of the universe, the
appearance of humans on the Earth, and the
voyage of Columbus to America. On January 1
of this imaginary year—at midnight—the big
bang takes place. Homo sapiens appears at
11:56 P.M. on December 31, and Columbus sets
sail on the last second of the last day of the
year. One second on this timescale is equivalent
to 500 true years.
1
Hydrogen
An electron is attracted by
and orbits the nucleus, which
has a proton and a neutron.
Proton
Neutron
Electron
NUCLEUS 2
NUCLEUS 1
2
Helium

Since the nucleus
has two protons,
two electrons are
attracted to it.
3
Carbon
With time, heavier and more complex elements
were formed. Carbon, the key to human life, has six
protons in its nucleus and six electrons orbiting it.
Quasar
Star
cluster
Nebula
Elliptical
galaxy
Irregular
galaxy
Star
Spiral
galaxy
Barred
spiral
galaxy
Galaxy
cluster
COLUMBUS'S
ARRIVAL
takes place on
the last second
of December 31.

THE SOLAR
SYSTEM
is created on
August 24 of
this timescale.
BIG BANG
occurs on the
first second of
the first day of
the year.
JANUARY DECEMBER
13.7 billion
-454° F (-270° C)
The universe continues to expand. Countless galaxies
are surrounded by dark matter, which represents 22
percent of the mass and energy in the universe. The
ordinary matter, of which stars and planets are
made, represents just 4 percent of the total. The predominant
form of energy is also of an unknown type. Called dark energy, it
constitutes 74 percent of the total mass and energy.
11
DARK MATTER
The visible objects in the
cosmos represent only a
small fraction of the total
matter within the universe.
Most of it is invisible even to
the most powerful
telescopes. Galaxies and their
stars move as they do

because of the gravitational
forces exerted by this
material, which astronomers
call dark matter.
THE UNIVERSE TODAY
TIMESCALE
The Transparent Universe
With the creation of atoms and overall cooling, the once opaque and
dense universe became transparent. Electrons were attracted by the
protons of hydrogen and helium nuclei, and together they formed atoms.
Photons (massless particles of light) could now pass freely through the
universe. With the cooling, radiation remained abundant but was no longer the
sole governing factor of the universe. Matter, through gravitational force, could
now direct its own destiny. The gaseous lumps that were present in this
process grew larger and larger. After 100 million years, they formed even
larger objects. Their shapes not yet defined, they constituted protogalaxies.
Gravitation gave shape to the first galaxies some 500 million years after the
big bang, and the first stars began to shine in the densest regions of these
galaxies. One mystery that could not be solved was why galaxies were
distributed and shaped the way they were. The solution that astronomers have
been able to find through indirect evidence is that there exists material called
dark matter whose presence would have played a role in galaxy formation.
9.1 billion
THE EARTH IS CREATED
Like the rest of the planets, the Earth is made of
material that remained after the formation of the solar
system. The Earth is the only planet known to have life.
EVOLUTION OF MATTER
What can be observed in the universe today is a great
quantity of matter grouped into galaxies. But that was not

the original form of the universe. What the big bang initially
produced was a cloud of uniformly dispersed gas. Just three
million years later, the gas began to organize itself into
filaments. Today the universe can be seen as a network of
galactic filaments with enormous voids between them.
1
Gaseous cloud
The first gases
and dust resulting
from the Big Bang
form a cloud.
2
First filaments
Because of the
gravitational pull of dark
matter, the gases joined
in the form of filaments.
3
Filament networks
The universe has
large-scale filaments
that contain millions
and millions of galaxies.
9 billion
-432° F (-258° C)
Nine billion years after the big
bang, the solar system
emerged. A mass of gas and
dust collapsed until it gave rise
to the Sun. Later the planetary system was

formed from the leftover material.
10
UNIVERSE 15
14
WHAT IS THE UNIVERSE?
Everything Comes to an End
T
he big bang theory helped solve the enigma of the early moments of the universe. What has yet to
be resolved is the mystery surrounding the future that awaits. To unravel this mystery, the total
mass of the universe must be known, but that figure has not yet been reliably determined. The
most recent observations have removed some of this uncertainty. It seems that the mass of the universe
is far too little to stop its expansion. If this is this case, the universe's present growth is merely the last
step before its total death in complete darkness.
Black hole
Universe 1
Universe 1
Universe 4
Universe 3
Black
hole
Universe 2
Object in three
dimensions
Object that changes
with time
Universe 3
New universe
Inflection point
DISCOVERIES
The key discovery that led to the big

bang theory was made in the early
1920s by Edwin Hubble, who
discovered that galaxies were moving
away from each other. In the 1940s,
George Gamow developed the idea
that the universe began with a
primordial explosion. A consequence
of such an event would be the
existence of background radiation,
which Arno Penzias and Robert
Wilson accidentally detected in the
mid-1960s.
There is a critical amount of mass
for which the universe would
expand at a declining rate without
ever totally stopping. The result of this
eternal expansion would be the existence of
an ever-increasing number of galaxies and
stars. If the universe were flat, we could
talk about a cosmos born from an explosion,
but it would be a universe continuing
outward forever. It is difficult to think
about a universe with these characteristics.
Flat Universe
BIG BANG
BIG
CRUNCH
HOW IT IS MADE UP
Dark energy is hypothesized to be
the predominant energy in the

universe. It is believed to speed up
the expansion of the universe.
BLACK HOLES
Some theorists believe
that, by entering a
black hole, travel
through space to
other universes might
be possible because of
antigravitational
effects.
1
The universe
expands violently.
2
The universe's
growth slows.
3
The universe collapses
upon itself, forming a
dense, hot spot.
1
The universe
continuously
expands and
evolves.
TIME
2
The universe's
expansion is

unceasing but
ever slower.
2
Expansion is
continuous and
pronounced.
3
Gravity is not
sufficient to bring a
complete stop to the
universe's expansion.
4
The universe
expands indefinitely.
1
Self-generated
Universes
A less widely accepted theory about
the nature of the universe suggests
that universes generate themselves.
If this is the case, universes would be
created continuously like the branches of a
tree, and they might be linked by
supermassive black holes.
According to this theory, universes
continuously sprout other universes. But
in this case, one universe would be
created from the death or disappearance of
another. Each dead universe in a final collapse, or
Big Crunch, would give rise to a supermassive

black hole, from which another universe would
be born. This process could repeat itself
indefinitely, making the number of universes
impossible to determine.
Baby Universes
5
Closed Universe
If the universe had more than
critical mass, it would expand
until reaching a point where
gravity stopped the expansion. Then,
the universe would contract in the Big
Crunch, a total collapse culminating in
an infinitely small, dense, and hot spot
similar to the one from which the
universe was formed. Gravity's pull on
the universe's excess matter would stop
the expansion and reverse the process.
2
Open Universe
The most accepted theory about
the future of the cosmos says
that the universe possesses a
mass smaller than the critical value. The
latest measurements seem to indicate that
the present time is just a phase before the
death of the universe, in which it goes
completely dark.
4
1

After the original
expansion, the
universe grows.
3
reaches a point where
everything grows dark
and life is extinguished.
3
74%
dark energy
22%
dark matter
4%
visible matter
GALACTIC EXPANSION
By noting a redshift toward the red end of the
spectrum, Hubble was able to demonstrate that
galaxies were moving away from each other.
1920s
GAMOW'S SUSPICION
Gamow first hypothesized the big bang,
holding that the early universe was a
“cauldron” of particles.
1940s
BACKGROUND RADIATION
Penzias and Wilson detected radio signals
that came from across the entire sky—the
uniform signal of background radiation.
1965
THE HAWKING UNIVERSE

The universe was composed originally of four
spatial dimensions without the dimension of
time. Since there is no change without time,
one of these dimensions, according to Hawking,
transformed spontaneously on a small scale
into a temporal dimension, and the universe
began to expand.
UNIVERSE 1716 WHAT IS THE UNIVERSE?
The Forces of the Universe
T
he four fundamental forces of nature are those that are not derived from basic forces. Physicists
believe that, at one time, all physical forces functioned as a single force and that during the
expansion of the universe, they became distinct from each other. Each force now governs different
processes, and each interaction affects different types of particles. Gravity, electromagnetism, strong
nuclear interactions, and weak nuclear interactions are essential to our understanding of the behavior of
the many objects that exist in the universe. In recent years, many scientists have tried with little success
to show how all forces are manifestations of a single type of exchange.
The universe, if it were empty, could be
pictured in this way.
The universe is deformed by the mass
of the objects it contains.
MOLECULAR MAGNETISM
In atoms and molecules, the electromagnetic force is
dominant. It is the force that causes the attraction
between protons and electrons in an atom and the
attraction or repulsion between ionized atoms.
NEWTON'S EQUATION
BENDING LIGHT
Light also bends because of the curvature of space-time.
When seen from a telescope, the real position of an object

is distorted. What is perceived through the telescope is a
false location, generated by the curvature of the light. It
is not possible to see the actual position of the object.
The biggest contribution to our comprehension of the universe's internal
workings was made by Albert Einstein in 1915. Building on Newton's
theory of universal gravitation, Einstein thought of space as linked to time. To
Newton, gravity was merely the force that attracted two objects, but Einstein
hypothesized that it was a consequence of what he called the curvature of space-
time. According to his general theory of relativity, the universe curves in the
presence of objects with mass. Gravity, according to this theory, is a distortion of
space that determines whether one object rolls toward another. Einstein's general
theory of relativity required scientists to consider the universe in terms of a non-
Euclidian geometry, since it is not compatible with the idea of a flat universe.
In Einsteinian space, two parallel lines can meet.
General Theory of Relativity
UNIVERSAL GRAVITATION
The gravitation proposed by Newton is
the mutual attraction between bodies
having mass. The equation developed by
Newton to calculate this force states
that the attraction experienced by two
bodies is directly proportional to the
product of their masses and inversely
proportional to the square of the
distance between them. Newton
represented the constant of
proportionality resulting from this
interaction as G. The shortcoming of
Newton's law, an accepted paradigm
until Einstein's theory of general

relativity, lies in its failure to make time
an essential component in the
interaction between objects. According
to Newton, the gravitational attraction
between two objects with mass did not
depend on the properties of space but
was an intrinsic property of the objects
themselves. Nevertheless, Newton's law
of universal gravitation was a
foundation for Einstein's theory.
The strong nuclear force holds the protons and neutrons
of atomic nuclei together. Both protons and neutrons are
subject to this force. Gluons are particles that carry the
strong nuclear force, and they bind quarks together to form
protons and neutrons. Atomic nuclei are held together by
residual forces in the interaction between quarks and gluons.
Strong Nuclear Force
3
The weak nuclear force is not as strong as the other
forces. The weak nuclear interaction influences the beta
decay of a neutron, which releases a proton and a
neutrino that later transforms into an electron. This force takes
part in the natural radioactive phenomena associated with certain
types of atoms.
Weak Nuclear Force
4
Electromagnetism is the force that affects
electrically charged bodies. It is involved in the
chemical and physical transformations of the
atoms and molecules of the various elements. The

electromagnetic force can be one of attraction or repulsion,
with two types of charges or poles.
Electromagnetism
2
Gravity was the first force to
become distinguishable from the
original superforce. Today
scientists understand gravity in Einstein's
terms as an effect of the curvature of
space-time. If the universe were thought of
as a cube, the presence of any object with
mass in space would deform the cube.
Gravity can act at great distances (just as
electromagnetism can) and always exerts a
force of attraction. Despite the many
attempts to find antigravity (which could
counteract the effects of black holes), it
has yet to be found.
Gravity
1
E=mc
2
In Einstein's equation, energy and mass are
interchangeable. If an object increases its
mass, its energy increases, and vice versa.
F=Gxm1xm2
d
2
SUN
EARTH

What
we see
LUMINOUS TRAJECTORY
Real
position
Two bodies with mass attract each other. Whichever
body has the greatest mass will exert a greater force
on the other. The greater the distance between the
objects, the smaller the force they exert on each other.
d
F
m1
m2
Quark
Force
Positive
pole
Positive
pole
Negative
pole
Negative
pole
Nucleus
Electron
Helium
Hydrogen
Neutron
WIMP
Nucleus

Proton
HYDROGEN ATOM
HELIUM ISOTOPE
Gluon
Force
Proton
Electron
Electron
1
Quarks and gluons
The strong nuclear interaction
takes place when the gluon
interacts with quarks.
Attraction
Two atoms are drawn together,
and the electrons rotate
around the new molecule.
1
Hydrogen
A hydrogen atom interacts
with a weak, light particle
(WIMP). A neutron's
bottom quark transforms
into a top quark.
2
Union
Quarks join and form
nuclear protons and
neutrons.
2

Helium
The neutron transforms
into a proton. An electron
is released, and the helium
isotope that is formed has
no nuclear neutrons.
THE FINAL DARKNESS 30-31
ANATOMY OF GALAXIES 32-33
ACTIVE GALAXIES 34-35
STELLAR METROPOLIS 36-37
What Is in the Universe?
T
he universe is populated on a
grand scale by strands of
superclusters surrounding
vacant areas. Sometimes the
galaxies collide with each
other, triggering the formation of stars.
In the vast cosmos, there are also
quasars, pulsars, and black holes.
Thanks to current technology, we can
enjoy the displays of light and shadow
that make up, for example, the Eta
Carinae Nebula (shown), which is
composed of jets of hot, fluorescent
gases. Although not all the objects in the
universe are known, it can be said
without a doubt that most of the atoms
that make up our bodies have been born
in the interior of stars.

LUMINOUS 20-21
STELLAR EVOLUTION 22-23
RED, DANGER, AND DEATH 24-25
GAS SHELLS 26-27
SUPERNOVAE 28-29
ETA CARINAE NEBULA
With a diameter of more than 200 light-years, it is
one of biggest and brightest nebulae of our galaxy.
This young, supermassive star is expected to become
a supernova in the near future.
100,000
SUN
Main
sequence
Supergiants
Red giants
White dwarfs
10,000
1,000
100
10
1
0.1
0.01
0.001
0.0001
INTRINSIC
LUMINOSITY (SUN = 1)
SPECTRAL CLASSES
OB A F G KM

UNIVERSE
2120
WHAT IS IN THE UNIVERSE?
Luminous
F
or a long time stars were a mystery to humans, and it was only as recently
as the 19th century that astronomers began to understand the true nature
of stars. Today we know that they are gigantic spheres of incandescent
gas—mostly hydrogen, with a smaller proportion of helium. As a star radiates
light, astronomers can precisely measure its brightness, color, and temperature.
Because of their enormous distance from the Earth, stars beyond the Sun only
appear as points of light, and even the most powerful telescopes do not reveal
any surface features.
COLORS The hottest stars are
bluish-white (spectral classes
O, B, and A). The coolest stars
are orange, yellow, and red
(spectral classes G, K, and M).
Calcium Hydrogen Hydrogen HydrogenSodium
Wavelength longest on the red side
When a star moves toward or away from an observer, its
wavelengths of light shift, a phenomenon called the Doppler effect.
If the star is approaching the Earth, the dark lines in its spectrum
experience a blueshift. If it moves away from the Earth, the lines
experience a redshift.
DOPPLER EFFECT
SCORPIUS REGION
0
1
2

3 4
5
6
7
8 9
10
11
12
13
14
15
16 17 18
19 20
21 22
23
24 26
27 28 29
30
PARSECS
PRINCIPAL STARS WITHIN 100 LY FROM THE SUN
SUN
(G2)
The H-R diagram plots the intrinsic
luminosity of stars against their
spectral class, which corresponds to their
temperature or the wavelengths of light
they emit. The most massive stars are
those with greatest intrinsic luminosity.
They include blue stars, red giants, and
red supergiants. Stars spend 90 percent

of their lives in what is known as the
main sequence.
Hertzsprung-Russell (H-R) Diagram
In measuring the great distances
between stars, both light-years (ly)
and parsecs (pc) are used. A light-year is
the distance that light travels in a year—
5.9 trillion miles (10 trillion km). A light-
year is a unit of distance, not time. A parsec
is equivalent to the distance between the
star and the Earth if the parallax angle is of
one second arc. A pc is equal to 3.26 light-
years, or 19 trillion miles (31 trillion km).
Light-years and Parsecs
When the Earth orbits the Sun, the closest stars
appear to move in front of a background of more
distant stars. The angle described by the movement of a
star in a six-month period of the Earth's rotation is called
its parallax. The parallax of the most distant stars are too
small to measure. The closer a star is to the Earth, the
greater its parallax.
Measuring Distance
The electromagnetic waves that make up light have
different wavelengths. When light from a hot
object, such as a star, is split into its different
wavelengths, a band of colors, or spectrum, is obtained.
Patterns of dark lines typically appear in the spectrum of
a star. These patterns can be studied to determine the
elements that make up the star.
Spectral Analysis

Dark lines deviate toward the blue end of the spectrum.
BLUESHIFT of a star moving toward the Earth.
OPEN CLUSTER
The Pleiades are a formation of
some 400 stars that will eventually
move apart.
GLOBULAR CLUSTER
More than a million stars are
grouped together into a spherical
cluster called Omega Centauri.
ALPHA
CENTAURI
(G2, K1, M5)
SIRIUS
(A0 and
dwarf star)
PROCYON
(F5 and
dwarf star)
ALTAIR
(A7)
VEGA
(A0)
POLLUX
(K0 giant)
ARCTURUS
(K2 giant)
CAPELLA
(G6 and G2
giants)

CASTOR
(A2, A1, and M1)
ALDEBARAN
(K5 giant)
ALIOTH
(A0 giant)
MENKALINAN
(A2 and A2)
ALGOL
(B8 and K0)
REGULUS
(B7 and K1)
25
0
1
2345
67
89
10 11 12 13 14 15 16 17 18
19
20 21 22 23 24 25
26
27 28 29 30 31
32
33
34 35 36 37 38
39 40 41 42
43
44
45

46 47 48 49
50
51 52
53
54 55 56 57 58
59
60 61 62 63
64
65 66 67 68 69 70 71 72 73 74 75 76 77 78
79
80
81
82
83
84 85 86 87
88
89
90
91 92 93 94
95
96 97 98 99 100
LIGHT-YEARS
GACRUX
(M4 giant)
TYPE O
52,000-72,000° F
(29,000-40,000° C)
TYPE B
17,500-52,000° F
(9,700-29,000° C)

TYPE A
13,000-17,500° F
(7,200-9,700° C)
TYPE F
10,500-13,000° F
(5,800-7,200° C)
TYPE G
8,500-10,500° F
(4,700-5,800° C)
TYPE K
6,000-8,500° F
(3,300-4,700° C)
TYPE M
4,000-6,000° F
(2,100-3,300° C)
Star Earth
Wavelength is compressed by
the movement of the star.
Because the parallax
of star A is small, we
see that it is distant
from the Earth.
Position of
the Earth in
January
Position of
the Earth in
July
PARALLAX
SUN

A
The parallax of star B
is greater than that of
star A, so we see that
B is closer to the
Earth.
B
A CLOUD OF GAS AND DUST collapses
because of gravitational forces. In doing
so it heats up and divides into smaller
clouds. Each one of these clouds will
form a protostar.
PROTOSTAR
A protostar has a
dense, gaseous
core surrounded by
a cloud of dust.
1.
RED SUPERGIANT
The star swells and heats up.
Through nuclear reactions, a
heavy core of iron is formed.
3.
NEUTRON STAR
If the star's initial mass is between
eight and 20 solar masses, it ends up
as a neutron star.
BLACK HOLE If the star's initial
mass is 20 solar masses or more, its
nucleus is denser and it turns into a

black hole, whose gravitational force
is extremely strong.
5.
5.
PROTOSTAR
A protostar is formed
by the separation of gas
and dust. Gravitational
effects cause its core to
rotate.
1.
PLANETARY NEBULA When
the star's fuel is depleted, its
core condenses, and its outer
layers detach, expelling
gases in an expanding shell
of gases.
WHITE DWARF
The star remains
surrounded by
gases and is dim.
5.
22
WHAT IS IN THE UNIVERSE? UNIVERSE
23
S
tars are born in nebulae, which are giant clouds of gas (mainly hydrogen)
and dust that float in space. Stars can have a life span of millions,
or even billions, of years. The biggest stars have the
shortest lives, because they consume their nuclear

fuel (hydrogen) at a very accelerated rate. Other
stars, like the Sun, burn fuel at a slower rate and
may live some 10 billion years. Many times, a
star's size indicates its age. Smaller stars are the
youngest, and bigger stars are approaching
their end, either through cooling or by
exploding as a supernova.
Stellar Evolution
Nebula
Small star
Less than 8 solar masses
STAR
A star is finally born. It
fuses hydrogen to form
helium and lies along
the main sequence.
2.
The evolution of a star depends on its mass. The
smallest ones, like the Sun, have relatively long and
modest lives. Such a star begins to burn helium when its
hydrogen is depleted. In this way, its external layers
begin to swell until the star turns into a red giant. It
ends its life as white dwarfs, eventually fading away
completely, ejecting remaining outer layers, and forming
a planetary nebula. A massive star, because of its higher
density, can form elements heavier than helium from its
nuclear reactions. In the final stage of its life, its core
collapses and the star explodes. All that remains is a
hyperdense remnant, a neutron star. The most massive
stars end by forming black holes.

Life Cycle of a Star
end their lives as white dwarfs. Other (larger)
stars explode as supernovae, illuminating
galaxies for weeks, although their brightness is
often obscured by the gases and dust.
STAR The star shines and
slowly consumes its
hydrogen. It begins to fuse
helium as its size increases.
2.
RED GIANT The star continues to
expand, but its mass remains
constant and its core heats up.
When the star's helium is depleted,
it fuses carbon and oxygen.
3.
Massive star
More than 8 solar masses
95% of stars
BLACK DWARF
If a white dwarf
fades out
completely, it
becomes a black
dwarf.
6.
4.
4.
SUPERNOVA When the star can no longer
fuse any more elements, its core collapses,

causing a strong emission of energy.
Red
giant
3
4
5
6
7
2
3
4
6
7
LIFE CYCLE OF A
STAR
5
W
hen a star exhausts its hydrogen, it begins to die. The
helium that now makes up the star's core begins to
undergo nuclear reactions, and the star remains
bright. When the star's helium is depleted, fusion of
carbon and oxygen begins, which causes the star's
core to contract. The star continues to live, though its
surface layers begin to expand and cool as the star turns
into a red giant. Stars similar to the Sun (solar-type stars)
follow this process. After billions of years, they end up as
white dwarfs. When they are fully extinguished, they will
be black dwarfs, invisible in space.
UNIVERSE
2524

WHAT IS IN THE UNIVERSE?
Red, Danger, and Death
DIAMETER
All stars go through a red-giant
stage. Depending on a star's
mass, it may collapse or it may simply
die enveloped in gaseous layers. The
core of a red giant is 10 times smaller
than it was originally since it shrinks
from a lack of hydrogen. A supergiant
star (one with an initial mass greater
than eight solar masses) lives a much
shorter life. Because of the high density
attained by its core, it eventually
collapses in on itself and explodes.
Red Giant
HERTZSPRUNG-RUSSELL
When a white dwarf leaves the
red-giant stage, it occupies the
lower-left corner of the H-R
diagram. Its temperature may be
double that of a typical red giant.
A massive white dwarf can
collapse in on itself and end its
life as a neutron star.
HYDROGEN
Hydrogen continues undergoing
nuclear fusion in the exterior of
the core even when the inner
core has run out of hydrogen.

HELIUM
Helium is produced by the fusion
of hydrogen during the main
sequence.
CARBON AND OXYGEN
Carbon and oxygen are produced
by the fusion of helium within the
core of the red giant.
1
2
3
SUN
Convection
Cells
Convection cells carry heat toward
the surface of a star. The ascending
currents of gas eventually reach
the surface of the star, carrying
with them a few elements that
formed in the star's core.
Hot Spots
Hot spots appear when large
jets of incandescent gas
reach the star's surface.
They can be detected on the
surface of red giants.
REGION OF THE CORE
TEMPERATURE
As the helium undergoes fusion,
the temperature of the core

reaches millions of degrees
Fahrenheit (millions of degrees
Celsius).
4
Venus's orbit
Mercury's orbit
Earth's orbit
Mars's orbit
Jupiter's orbit
Saturn's orbit
Red supergiant. Placed
at the center of the
solar system, it would
swallow up Mars and
Jupiter.
Red giant. Placed at
the center of the solar
system, it could reach
only the nearer planets,
such as Mercury,
Venus, and the Earth.
1%
The scale of the
diameter of the Sun
to the diameter of
a typical red giant.
After going through the red-giant stage, a solar-type star loses its
outer layers, giving rise to a planetary nebula. In its center remains a
white dwarf—a relatively small, very hot (360,000° F [200,000° C]), dense
star. After cooling for millions of years, it shuts down completely and

becomes a black dwarf.
White Dwarf
On leaving the main sequence,
the star enlarges to 200 times
the size of the Sun. When the
star begins to burn helium, its
size decreases to between 10 and
100 times the size of the Sun.
The star then remains stable until
it becomes a white dwarf.
SPECTACULAR DIMENSIONS
HERTZSPRUNG-RUSSELL
When the star exhausts its
hydrogen, it leaves the main
sequence and burns helium
as a red giant (or a
supergiant). The smallest
stars take billions of years to
leave the main sequences.
The color of a red giant is
caused by its relatively cool
surface temperature of
3,600° F (2,000° C).
Sun
NEBULa NGC 6751
After the nuclear reaction in the star's core ceases, the star
ejects its outer layers, which then form a planetary nebula.
WHITE DWARF
Mars
Venus

Sun
Earth
Mercury
Mars
Venus
Sun
Earth Mercury
Mars
Venus
Sun
Earth
Like any typical star, the Sun burns hydrogen
during its main sequence. After taking
approximately five billion years to exhaust its
supply of hydrogen, it will begin its
transformation into a red giant, doubling in
brightness and expanding until it swallows
Mercury. At its maximum size, it may even
envelop the Earth. Once it has stabilized, it will
continue as a red giant for two billion years and
then become a white dwarf.
THE FUTURE OF THE SUN
Dust Grains
Dust grains condense in the star's outer
atmosphere and later disperse in the form of
stellar winds. The dust acquires a dark
appearance and is swept into interstellar
space, where new generations of stars will
form. The outer layer of the star may
extend across several light-years of

interstellar space.
RED GIANT
The radius of the
Sun reaches the
Earth's orbit.
Earth
1
2
Gas Shells
HOURGLASS
HELIX
SPIROGRAPH
W
hen a small star dies, all that remains is an expanding
gas shell known as a planetary nebula, which has
nothing to do with the planets. In general,
planetary nebulae are symmetrical or spherical
objects. Although it has not been possible to
determine why they exist in such diversity, the reason
may be related to the effects of the magnetic field of the
dying central star. Viewed through a telescope, several
nebulae can be seen to contain a central dwarf star, a mere
remnant of its precursor star.
UNIVERSE
27
The two rings of colored
gas form the silhouette of
this hourglass-shaped
nebula. The red in the
photograph corresponds to

nitrogen, and the green
corresponds to hydrogen.
This nebula is 8,000 light-
years from the Earth.
MYCN 18
NGC 7293
BUTTERFLY
The density of a white dwarf is a million
times greater than the density of water.
In other words, each cubic meter of a
white dwarf star weighs a million tons.
The mass of a star is indirectly
proportional to its diameter. A white
dwarf with a diameter 100 times smaller
than the Sun has a mass 70 times greater.
M2-9
The Butterfly Nebula contains
a star in addition to a white
dwarf. Each orbits the other
inside a gas disk that is 10
times larger than Pluto's
orbit. The Butterfly Nebula
is located 2,100 light-years
from Earth.
IC 418
The Spirograph Nebula has a
hot, luminous core that
excites nearby atoms,
causing them to glow. The
Spirograph Nebula is about

0.1 light-year wide and is
located 2,000 light-years
from Earth.
White
Dwarf
The remains of the red
giant, in which the
fusion of carbon and
oxygen has ceased, lie
at the center of the
nebula. The star slowly
cools and fades.
Hydrogen
The continuously expanding
masses of gas surrounding the
star contain mostly hydrogen,
with helium and lesser amounts
of oxygen, nitrogen, and other
elements.
Concentric
circles
of gas, resembling the inside of an onion, form a
multilayered structure around the white dwarf.
Each layer has a mass greater than the combined
mass of all the planets in the solar system.
TWICE THE
TEMPERATURE OF
THE SUN
is reached at the surface of a
white dwarf, causing it to

appear white even though its
luminosity is a thousand times
less than that of the Sun.
is the weight of a single
tablespoon of a white
dwarf. A white dwarf is
very massive in spite of
the fact that its
diameter of 9,300 miles
(or 15,000 km) is
comparable to the
Earth's.
The astrophysicist
Subrahmanyan
Chandrasekhar, winner of the
Nobel Prize for Physics in
1983, calculated the maximum
mass a star could have so that
it would not eventually collapse
on itself. If a star's mass exceeds
this limit, the star will eventually
explode in a supernova.
CHANDRASEKHAR
LIMIT
1.44 SOLAR MASSES
is the limit Chandrasekhar
obtained. In excess of this value,
a dwarf star cannot support its
own gravity and collapses.
NGC 6542 CAT'S EYE

26
WHAT IS IN THE UNIVERSE?
SMALLER DIAMETER
More massive white dwarf
LARGER DIAMETER
Less massive white dwarf
DENSITY OF A WHITE DWARF
The Helix is a
planetary nebula that
was created at the
end of the life of a
solar-type star. It is
650 light-years from
the Earth and is
located in the
constellation Aquarius.
Planetary
nebula
1
2
3
4
5
6
7
2
3
4
6
7

LIFE CYCLE OF
A STAR
5
3 tons
28
WHAT IS IN THE UNIVERSE?
Supernovae
A
supernova is an extraordinary explosion of a giant star at
the end of its life, accompanied by a sudden increase
in brightness and the release of a great amount
of energy. In 10 seconds, a supernova releases 100
times more energy than the Sun will release in its
entire life. After the explosion of the star that gives
rise to a supernova, the gaseous remnant expands and
shines for millions of years. It is estimated that, in our Milky
Way galaxy, two supernovae occur per century.
Supernova
GAS AND DUST
Gas and dust that have
accumulated in the two visible lobes
absorb the blue light and ultraviolet
rays emitted from its center.
FUSION
The nuclear
reactions in a
dying star occur
at a faster rate
than they do in a
red giant.

GASEOUS FILAMENTS
Gaseous filaments are ejected
by the supernova at 620 miles
(1,000 km) per second.
THE END
Either a neutron star
or a black hole may
form depending on the
initial mass of the star
that has died.
Stellar Remnant
When the star explodes as a supernova, it
leaves as a legacy in space the heavy
elements (such as carbon, oxygen, and iron) that
were in the star's nucleus before its collapse. The
Crab Nebula (M1) was created by a supernova
seen in 1054 by Chinese astronomers. The Crab
Nebula is located 6.5 light-years from Earth and
has a diameter of six light-years. The star that
gave rise to the Crab Nebula may have had an
initial mass close to 10 solar masses. In 1969, a
pulsar radiating X-rays and rotating 33 times per
second was discovered at the center of the
nebula, making the Crab Nebula a very powerful
source of radiation.
The explosion that marks the
end of a supergiant's life occurs
because the star's extremely heavy
core has become incapable of
supporting its own gravity any longer.

In the absence of fusion in its interior,
the star falls in upon itself, expelling
its remaining gases, which will expand
and shine for hundreds—or even
thousands—of years. The explosion of
the star injects new material into
interstellar space and contributes
heavy atoms that can give rise to new
generations of stars.
The Twilight of a Star
Supergiant
The diameter of the star may
increase to more than 1,000
times that of the Sun. Through
nuclear fusion, the star can
produce elements even heavier
than carbon and oxygen.
When a star's iron core
increases in density to 1.44
solar masses, the star can
no longer support its own
weight and it collapses
upon itself. The resulting
explosion causes the
formation of elements that
are heavier than iron, such
as gold and uranium.
Other Elements
Core
A star's core can be seen to

be separated into distinct
layers that correspond to
the different elements
created during nuclear
fusion. The last
element created
before the star's
collapse is iron.
DENSE
CORE
CRAB NEBULA
Explosion
The star's life ends in an immense
explosion. During the weeks
following the explosion, great
quantities of energy are radiated
that are sometimes greater than the
energy emitted by the star's parent
galaxy. A supernova may illuminate
its galaxy for weeks.
ETA CARINAE
SUPERMASSIVE
The mass of Eta Carinae
is 100 times greater
than that of the Sun.
Astronomers believe
that Eta Carinae is
about to explode,
but no one knows
when.

The image at left shows a sector of
the Large Magellanic Cloud, an
irregular galaxy located 170,000 light-
years from the Earth, depicted before
the explosion of supernova 1987A. The
image at right shows the supernova.
BEFORE AND AFTER
FEBRUARY
23, 1987
After the supernova
explosion, increased
brightness is
observed in the
region near the star.
FEBRUARY 22,
1987
This star is in its last
moments of life. Because it
is very massive, it will end
its life in an explosion. The
galaxy exhibits only its usual
luminosity.
LIFE CYCLE OF
A STAR
1
2
3
4
5
6

7
2
3
4
6
7
5
UNIVERSE
3130
WHAT IS IN THE UNIVERSE?
The Final Darkness
T
he last stage in the evolution of a star's core is its
transformation into a very dense, compact stellar body.
Its particulars depend upon the amount of mass
involved in its collapse. The largest stars become black
holes, their density so great that their gravitational
forces capture even light. The only way to detect these
dead stars is by searching for the effects of their
gravitation.
Discovery of Black Holes
The only way of detecting the presence of
a black hole in space is by its effect on
neighboring stars. Since the gravitational force
exerted by a black hole is so powerful, the gases
of nearby stars are absorbed at great speed,
spiraling toward the black hole and forming a
structure called an accretion disk. The friction
of the gases heats them until they shine
brightly. The hottest parts of the accretion disk

may reach 100,000,000° C and are a source of
X-rays. The black hole, by exerting such
powerful gravitational force, attracts everything
that passes close to it, letting nothing escape.
Since even light is not exempt from this
phenomenon, black holes are opaque and
invisible to even the most advanced telescopes.
Some astronomers believe that
supermassive black holes might have
a mass of millions, or even
billions, of solar masses.
When a star's initial mass is between
10 and 20 solar masses, its final mass
will be larger than the mass of the Sun.
Despite losing great quantities of matter
during nuclear reactions, the star finishes
with a very dense core. Because of its intense
magnetic and gravitational fields, a neutron
star can end up as a pulsar. A pulsar is a
rapidly spinning neutron star that gives off a
beam of radio waves or other radiation. As
the beam sweeps around the object, the
radiation is observed in very regular pulses.
tons is what one tablespoon of a
neutron star would weigh. Its small
diameter causes the star to have a
compact, dense core accompanied by
intense gravitational effects.
1 billion
RED GIANT

A red giant leaves
the main sequence.
Its diameter is 100
times greater than
the Sun's.
1
SUPERGIANT
A supergiant grows
and rapidly fuses
heavier chemical
elements, forming
carbon, oxygen, and
finally iron.
2
EXPLOSION
The star's iron core
collapses. Protons
and electrons
annihilate each other
and form neutrons.
3
DENSE CORE
The core's exact
composition is
presently unknown.
Most of its
interacting particles
are neutrons.
4
Pulsars

The first pulsar (a neutron star radiating
radio waves) was discovered in 1967.
Pulsars rotate approximately 30 times per second
and have very intense magnetic fields. Pulsars
emit radio waves from their two magnetic poles
when they rotate. If a pulsar absorbs gas from a
neighboring star, a hot spot that radiates X-rays
is produced on the pulsar's surface.
Devouring gas from
a supergiant
Located within a binary system, the pulsar can
follow the same process as a black hole. The pulsar's
gravitational force causes it to absorb the gas of
smaller, neighboring stars, heating up the pulsar's
surface and causing it to emit X-rays.
CURVED SPACE
THE SUN forms a shallow
gravitational well.
1
A WHITE
DWARF
generates a
deeper
gravitational
well, drawing
in objects at a
higher speed.
2
Accretion Disk
An accretion disk is a gaseous accumulation

of matter that the black hole draws from
nearby stars. In the regions of the disk
very close to the black hole, X-rays are
emitted. The gas that accumulates
rotates at very high speeds. When
the gases from other stars
collide with the disk, they
create bright, hot spots.
Bright gases
Since the accretion disk is fed by gases
spinning at high speed, it shines
intensely in the region closest to its core
but at its edges is colder and darker.
Rotation axis
Radio-wave beam
Magnetic field
Possible
solid core
Neutron
star
Strong Gravitational
Attraction
The gravitational force of the black hole attracts
gases from a neighboring star. This gas forms a
large spiral that swirls faster and faster as it gets
closer to the black hole. The gravitation field that
it generates is so strong that it traps objects that
pass close to it.
LIGHT RAYS
BLACK HOLE

The objects that approach
the black hole too closely
are swallowed by it. The black hole's
gravitational well is infinite and traps
matter and light forever. The event
horizon describes the limit of what is,
and is not, absorbed. Any object that
crosses the event horizon follows a spiral
path into the gravitational well. Some
scientists believe in the existence of so-
called wormholes—antigravity tunnels,
through which travel across the universe
is hypothesized to be possible. By taking
advantage of the curvature of space, scientists
think it could be possible to travel from the
Earth to the Moon in a matter of seconds.
4
WORMHOLE
ENTRANCE
EXIT
Neutron Star
STRUCTURE OF A PULSAR
X-RAYS
As gases enter the
black hole, they are
heated and emit X-rays.
Total escape
Rays of light that
pass far from the
center of a black

hole continue
unaffected.
A NEUTRON STAR
attracts objects at
speeds approaching half
the speed of light. The
gravitational well is even
more pronounced.
3
The theory of relativity suggests
that gravity is not a force but a
distortion of space. This distortion
creates a gravitational well, the
depth of which depends on the
mass of the object. Objects are
attracted to other objects through
the curvature of space.
Close to the limit
Since the rays of light
have not crossed the
event horizon, they
still retain their
brightness.
Darkness
Rays of light that
pass close to the
core of a black hole
are trapped.
1
2

3
4
5
7
2
3
4
6
7
LIFE CYCLE OF
A STAR
5
6
Black hole
Neutron star
CROSS SECTION
HOT
GASES
ACCRETION
DISK
X-RAYS
BLACK
HOLE
LOSS OF MASS
Toward the end of
its life, a neutron
star loses more than
90 percent of its
initial mass.
Star Cities

The first galaxies formed 100 million years
after the big bang. Billions of these great
conglomerates of stars can be found throughout
space. The two most important discoveries
concerning galaxies are attributed to the
astronomer Edwin Hubble. In 1926, he pointed out
that the spots, or patches, of light visible in the
night sky were actually distant galaxies. Hubble's
discovery put an end to the view held by astronomers
at the time that the Milky Way constituted the
universe. In 1929, as a result of various observations
of the spectrum of light radiated by the stars in the
galaxies, Hubble noted that the light from the galaxies
showed a redshift (Doppler effect). This effect
indicated that the galaxies were moving away from
the Milky Way Galaxy. Hubble concluded that the
32
WHAT IS IN THE UNIVERSE? UNIVERSE
33
Galactic Clusters
Galaxies are objects that tend to form groups or clusters.
Acting in response to gravitational force, they can form
clusters of galaxies of anywhere from two to thousands of
galaxies. These clusters have various shapes and are thought to
expand when they join together. The Hercules cluster, shown here,
was discovered by Edmond Halley in 1714 and is located
approximately 25,100 light-years from Earth. Each dot represents
a galaxy that includes billions of stars.
Anatomy of Galaxies
COLLISION

300 million light-years
from the Earth, these
two colliding galaxies
form a pair. Together
they are called “The
Mice” for the large tail
of stars emanating
from each galaxy. With
time, these galaxies will
fuse into a single, larger
one. It is believed that
in the future the
universe will consist of
a few giant stars.
MILKY WAY
Seen from its side, the Milky Way looks like a
flattened disk, swollen at the center. Around
the disk is a spherical region, called a halo,
containing dark matter and globular clusters
of stars. From June to September, the Milky
Way is especially bright, something that
would make it more visible viewed from above
than from the side.
1.2 BILLION
YEARS
ago, the Antennae
(NGC 4038 and
NGC 4039) were
two separate
spiral galaxies.

1
300 MILLION
YEARS
later, the
galaxies
collided at
great speed.
2
300 MILLION
YEARS
go by until the
collision takes
place and the
shapes of the
galaxies are
distorted.
3
300 MILLION
YEARS
later, the
stars in the
spiral arms
are expelled
from both
galaxies.
4
NOW
two jets of
expelled
stars stretch

far from the
original
galaxies.
5
universe is expanding. But the expansion of the
universe does not imply that galaxies are growing in
numbers. On the contrary, galaxies can collide and
merge. When two galaxies collide, they can distort
each other in various ways. Over time, there are fewer
and fewer galaxies. Some galaxies exhibit very peculiar
ELLIPTICAL
These galaxies are elliptical in
shape and have little dust and
gas. Their masses fall within a
wide range.
SPIRAL
In a spiral galaxy, a nucleus of
old stars is surrounded by a flat
disk of stars and two or more
spiral arms.
Galaxies are subdivided into
different categories according to
their tendency toward round
shape (in the case of elliptical
galaxies), as well as by the
presence of an axis and the length
of their arms (in the case of spiral
and barred spiral galaxies). An E0
galaxy is elliptical but almost
circular, and an E7 galaxy is a

flattened oval. An Sa galaxy has a
large central axis and coiled arms,
and an Sc galaxy has a thinner axis
and more extended arms.
IRREGULAR
Irregular galaxies have no defined
shape and cannot be classified.
They contain a large amount of
gases and dust clouds.
ngc 4676
CLASSIFYING GALAXIES ACCORDING TO HUBBLE
SUBCLASSIFICATIONS
ngc 6205 hercules
shapes. The Sombrero Galaxy, shown in the center of
the page, has a bright white core surrounded by thin
spiral arms.
G
alaxies are rotating groups of stars,
gas, and dust. More than 200 years ago,
philosopher Immanuel Kant postulated
that nebulae were island-universes of distant
stars. Even though astronomers now know that
galaxies are held together by gravitational force,
they have not been able to decipher what reasons
might be behind galaxies' many shapes. The various
types of galaxies range from ovals of old stars to spirals
with arms of young stars and bright gases. The center of a
galaxy has the greatest accumulation of stars. The Milky Way
Galaxy is now known to be so big that rays of light, which travel at
186,000 miles (300,000 km) per second, take 100,000 years to cross

from one end to the other.
1
A
small number of galaxies differ from the rest by emitting high amounts of
energy. The energy emission might be caused by the presence of black holes in
its core that were formed through the gravitational collapse accompanying the
death of supermassive stars. During their first billion years, the galaxies might have
accumulated surrounding gaseous disks with their corresponding emissions of radiation. It
is possible that the cores of the first galaxies are the quasars that are now observed at very
great distances.
The Force
of Gravity
Gravitational force begins to
unite vast quantities of hot,
gaseous clouds. The clouds
attract one another and collide,
forming stars. A large amount of
gas accumulates at the center of
the galaxy, intensifying
gravitational forces until a
massive black hole comes into
being in the galaxy's core.
2
The Quasar
in the Core
The quasar in the core ejects two
jets of particles that reach
speeds approaching the speed of
light. The quasar stage is thought
to have been the most violent

stage in the formation of
galaxies. The gases and stars
arising from the jets are
introduced as spirals into the
black hole, forming a type of
accretion disk known as a quasar.
4
Stable Galaxy
Nine billion years after its formation, with a
supermassive black hole at its core, the galaxy
drastically slows its energetic activity, forming a low-
energy core. The stabilization of the galaxy allowed
the formation of stars and other heavenly bodies.
Active Galaxies
Astronomers believe that active galaxies are
a direct legacy from the beginning of the
universe. After the big bang, these galaxies would
have retained very energetic levels of radiation.
Quasars, the brightest and most ancient objects in
the universe, make up the core of this type of
galaxy. In some cases, they emit X-rays or radio
waves. The existence of this high-energy activity
helps support the theory that galaxies could be
born from a supermassive black hole with a
quasar that became inactive as stars formed and
it was left without gas to feed it. This process
of formation might be common to many
galaxies. Today quasars represent the
limit of what it is possible to see,
even with specialized

telescopes. Quasars are
small, dense, and bright.
Energetic Activity
A theory of galaxy formation
associated with active galaxies
holds that many galaxies, possibly
including the Milky Way, were formed
from the gradual calming of a quasar
at their core. As the surrounding
gases consolidated in the formation of
stars, the quasars, having no more
gases to absorb, lost their energetic
fury and became inactive. According
to this theory, there is a natural
progression from quasars to active
galaxies to the common galaxies of
today. In 1994, astronomers studying
the center of the Milky Way
discovered a region that may contain
a black hole and could be left over
from early galactic activity.
Galaxy Formation
GAS
As two jets are expelled from
the core, radio waves are
emitted. If the waves
collide with clouds of
intergalactic gas, they
swell and form
gigantic clouds that

can emit radio
waves or X-rays.
CENTRAL RING
The core of an active
galaxy is obscured
by a ring of dust
and gas that is
dark on the
outside and
bright within.
It is a powerful
source of
energy.
GASEOUS CLOUDS
Gaseous clouds appeared from the
gravitational collapse of immense masses
of gas during the early stages of the
universe. Later, in the clouds' interior,
stars began to form.
3
Black Hole
A black hole swallows the gas that begins to
surround it. A hot, gaseous spiral forms,
emitting high-speed jets. The magnetic field
pours charged particles into the region around
the black hole, and the exterior of the disk
absorbs interstellar gas.
UNIVERSE
3534
WHAT IS IN THE UNIVERSE?

The classification of an active galaxy depends
upon its distance from Earth and the perspective
from which it is seen. Quasars, radio galaxies, and
blazars are members of the same family of objects
and differ only in the way they are perceived.
CLASSIFICATION
QUASARS The most
powerful objects in the
universe, quasars are so distant
from Earth that they appear to us
as diffuse stars. They are the
bright cores of remote galaxies.
RADIO GALAXIES Radio galaxies are
the largest objects in the universe. Jets of
gases come out from their centers that
extend thousands of light-years. The cores
of radio galaxies cannot be seen.
BLAZARS Blazars may be
active galaxies with jets of gas
that are aimed directly toward
Earth. The brightness of a blazar
varies from day to day.
PARTICLES
ejected from the black hole
have intense magnetic fields.
The jets of particles travel at
speeds approaching the speed of
light when they leave the core.
Dark clouds of gas
and dust on the

outer edge of a
black hole are
gradually
swallowed up.
ACCRETION
DISK
Formed by interstellar
gas and star remnants,
the accretion disk can
radiate X-rays because of
the extreme temperature of
its center.
100
MILLION DEGREES
Celsius is the temperature that the
core of a black hole can reach.
4
The center of the
black hole radiates
charged particles.
3
The strong
gravitational force of
the disk attracts and
destroys stars.
2
As the gases
move inward,
their temperature
increases.

1
INCREASING
GRAVITY
UNIVERSE
3736
WHAT IS IN THE UNIVERSE?
Stellar Metropolis
F
or a long time, our galaxy (called the Milky Way because of
its resemblance to a stream of milk in the night sky) was a
true enigma. It was Galileo Galilei who, in 1610, first pointed
a telescope at the Milky Way and saw that the weak whitish strip
was composed of thousands and thousands of stars that appeared
to almost touch each other. Little by little, astronomers began to
realize that all these stars, like our own Sun, were part of the enormous
ensemble—the galaxy that is our stellar metropolis.
GASES SWIRL
outward because of forces in the
Sagittarius A region. Because the
gas rotates at high speed but
remains concentrated, it could be
trapped by gravitational forces
exerted by a black hole.
BRIGHT STARS
Bright stars are
born from gas that
is not absorbed by
the black hole. Most
of them are young.
Central Region

Because the Milky Way is full of clouds of dust and rock
particles, its center cannot be seen from outside the galaxy.
The Milky Way's center can be seen only through telescopes
that record infrared light, radio waves, or X-rays, which can
pass through the material that blocks visible light. The
central axis of the Milky Way contains ancient stars, some 14
billion years old, and exhibits intense activity within its
interior, where two clouds of hot gas have been found:
Sagittarius A and B. In the central region, but outside the
core, a giant dark cloud contains 70 different types of
molecules. These gas clouds are associated with violent
activity in the center of our galaxy and contain the heart of
the Milky Way within their depths. In general, the stars in
this region are cold and range in color from red to orange.
The core of the Milky Way galaxy is marked
by very intense radio-wave activity that
might be produced by an accretion disk
made up of incandescent gas surrounding a
massive black hole. The region of Sagittarius
A, discovered in 1994, is a gas ring that
rotates at very high speed, swirling within
several light-years of the center of the
galaxy. The speed of its rotation is an
indication of the powerful gravitational
force exerted from the center of the Milky
Way, a force stronger than would be
produced by the stars located in the region.
The hot, blue stars that shine in the center
of the Milky Way may have been born from
gas not yet absorbed by the black hole.

The Exact Center
The brightest portion of the Milky Way
that appears in photographs taken with
optical lenses (using visible light) is in the
constellation Sagittarius, which appears to lie in
the direction of the center of the Milky Way. The
bright band in the nighttime sky is made up of
stars so numerous that it is almost impossible to
count them. In some cases, stars are obscured
by dense dust clouds that make some regions of
the Milky Way seem truly dark. The objects that
can be found in the Milky Way are not all of one
type. Some, such as those known as the halo
population, are old and are distributed within a
sphere around the galaxy. Other objects form a
more flattened structure called the disk
population. In the spiral arm population, we find
the youngest objects in the Milky Way. In these
arms, gas and interstellar dust abound.
A Diverse Galaxy
ROTATION
The speeds of the rotation of the various parts of the
Milky Way vary according to those parts' distances from
the core of the galaxy. The greatest number of stars is
concentrated in the region between the Milky Way's
core and its border. Here the speed of rotation is much
greater because of the attraction that the objects in this
region feel from the billions of stars within it.
THE MILKY WAY IN VISIBLE LIGHT
THE CONSTELLATION

SAGITTARIUS
Close to the center of
the Milky Way,
Sagittarius shines
intensely.
SECTORS
Many different
sectors make up the
Milky Way.
STARS
So many
stars
compose the
Milky Way that it
is impossible for us
to distinguish them all.
DARK REGIONS
Dark regions are
produced by dense
clouds that obscure
the light
of stars.
HOT GASES
The hot gases originating
from the surface of the
central region may be the
result of violent explosions
in the accretion disk.
BLACK HOLE
Many astronomers believe that

a black hole occupies the
center of the Milky Way. Its
strong gravitational force
would trap gases in
orbit around it.
MAGNETISM
The center of the Milky
Way is surrounded by
strong magnetic fields,
perhaps from a rotating
black hole.
SAGITTARIUS B2
The largest dark cloud in
the central region of the
Milky Way, Sagittarius B2
contains enough alcohol to
cover the entire Earth.
OUTER RING
A ring of dark clouds of dust and
molecules that is expanding as a
result of a giant explosion. It is
suspected that a small object in
the central region of the Milky
Way might be its source.
Andromeda
Galaxy
MILKY WAY
The Milky Way, containing more than 100 billion stars, has two spiral arms
rotating around its core. The Sagittarius arm, located between the Orion arm
and the center of the Milky Way, holds one of the most luminous stars in the galaxy,

Eta Carinae. The Perseus arm, the main outer arm of the Milky Way, contains young
stars and nebulae. The Orion arm, extending between Perseus and Sagittarius,
houses the solar system within its inner border. The Orion arm of the Milky
Way is a veritable star factory, where gaseous
interstellar material can give birth to
billions of stars. Remnants of
stars can also be found
within it.
100,000
LIGHT-YEARS
The diameter of the Milky Way is
large in comparison with other
galaxies but not gigantic.
Central
protuberance
Triangle
Galaxy
Eagle
Nebula
Cassiopeia A
Eta
Carinae
6,000 light-years
Crab
Nebula
Orion
Nebula
Large
Magellanic Cloud
Small

Magellanic
Cloud
Structure of the Milky Way
30
0
60
0
90
0
120
0
150
0
180
0
210
0
240
0
270
0
360
0
0
0
SOLAR SYSTEM
1
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PLUTO: NOW A DWARF 58-59
DISTANT WORLDS 60-61
CONSTRUCTION DEBRIS:
ASTEROIDS AND METEORITES 62-63
THOSE WITH A TAIL 64-65
The Solar System
A
mong the millions and
millions of stars that form the
Milky Way galaxy, there is a
medium-sized one located in
one of the galaxy's arms—the
Sun. To ancient peoples, the Sun was a
god; to us, it is the central source of
energy that generates heat, helping life
exist. This star, together with the planets
and other bodies that spin in orbits
around it, make up the solar system,
which formed about 4.6 billion years ago.
The planets that rotate around it do not
produce their own light. Instead, they
reflect sunlight. After the Earth, Mars is
the most explored planet. Here we see a
photo of Olympus Mons, the largest
volcano in the entire solar system. It is
almost two-and-a-half times as high as the
tallest peak on the Earth, Mount Everest.
JUPITER, GAS GIANT 50-51

THE LORD OF THE RINGS 52-53
URANUS WITHOUT SECRETS 54-55
NEPTUNE: DEEP BLUE 56-57
ATTRACTED BY A STAR 40-41
A VERY WARM HEART 42-43
MERCURY, AN INFERNO 44-45
VENUS, OUR NEIGHBOR 46-47
RED AND FASCINATING 48-49
OLYMPUS MONS, ON MARS
Olympus Mons is the largest
volcano of the solar system. It
is about two-and-a-half times
as high as Mount Everest.
Outer Planets
Planets located outside the asteroid belt. They are enormous gas spheres with
small solid cores. They have very low temperatures because of their great
distance from the Sun. The presence of ring systems is exclusive to these planets. The
greatest of them is Jupiter: 1,300 Earths could fit inside of it. Its mass is 2.5 times as
great as that of the rest of the planets combined.
Inner Planets
Planets located inside the asteroid
belt. They are solid bodies in which
internal geologic phenomena, such as
volcanism, which can modify their surfaces,
are produced. Almost all of them have an
appreciable atmosphere of some degree of
thickness, according to individual
circumstances, which plays a key role in
the surface temperatures of each planet.
Asteroid Belt

The border between the outer and inner
planets is marked by millions of rocky
fragments of various sizes that form a band
called the asteroid belt. Their orbits are
influenced by the gravitational pull exerted on
them by the giant planet Jupiter. This effect also
keeps them from merging and forming a planet.
ORIGIN
Remains from the
formation of the Sun
created a disk of gas
and dust around it,
from which the
planetesimals formed.
Early ideas suggested that the planets formed gradually,
beginning with the binding of hot dust particles. Today
scientists suggest that the planets originated from the
collision and melding of larger-sized bodies called
planetesimals.
1
COLLISION
Through collisions
among themselves,
planetesimals of
different sizes joined
together to become
more massive objects.
2
HEAT
The collisions

produced a large
amount of heat that
accumulated in the
interior of the planets,
according to their
distance from the Sun.
3
BUILDING PLANETS
ORBITS
NEPTUNE
DIAMETER
MOONS
30,775 MILES (49,528 KM)
13
URANUS
DIAMETER
MOONS
31,763 MILES (51,118 KM)
27
SATURN
DIAMETER
MOONS
74,898 MILES (120,536 KM)
50+
JUPITER
DIAMETER
MOONS
88,846 MILES (142,984 KM)
60+
SUN

EARTH
DIAMETER
MOONS
7,926 MILES
(12,756 KM)
1
VENUS
DIAMETER
MOONS
7,520 MILES
(12,103 KM)
0
Venus's
orbit
Jupiter's
orbit
Saturn's
orbit
Uranus's
orbit
Neptune's
orbit
Earth's
orbit
Mercury's
orbit
Mars's
orbit
Main
belt

In general, the
planets orbit in one
common plane
called the elliptic.
The rotation of most planets around their own
axes is in counterclockwise direction. Venus and
Uranus, however, revolve clockwise.
Triton
Titania Oberon
Titan Rhea Iapetus Tethys
Ganymede Callisto Io Europa
Umbriel Ariel Miranda Puck
Nereid
MOON
Phobos Deimos
SOLAR
GRAVITY
MARS
DIAMETER
MOONS
4,217 MILES
(6,786 KM)
2
DIAMETER
MOONS
3,031 MILES
(4,878 KM)
0
MERCURY
Proteus

P
lanets and their satellites, asteroids and other rocky
objects, and an incalculable number of cometlike objects,
some more than 1 trillion miles (1.6 trillion km) from the
Sun, make up the solar system. In the 17th century, astronomer
Johannes Kepler proposed a model to interpret the dynamic
properties of the bodies of the solar system. According to this
interpretation, the planets complete elliptical trajectories, called
orbits, around the Sun. In every case, the movement is produced
by the influence of the gravitational field of the Sun. Today, as
part of a rapidly developing field of astronomy, it is known that
planet or planetlike bodies also orbit other stars.
Attracted by a Star
40
THE SOLAR SYSTEM
UNIVERSE
41
The gravitational pull of the
Sun upon the planets not
only keeps them inside
the solar system but
also influences the
speed with which
they revolve in their
orbits around the Sun.
Those closest to the Sun
revolve in their orbits much
faster than those farther
from it.